Synthetic hydrocarbon lubricants obtained from the Friedel-Crafts catalyzed oligomerization of .alpha.-olefins having from about 4 to 12 carbon atoms, particularly 1-octene and/or 1-decene, are known. For example, U.S. Pat. Nos. 3,149,178; 3,763,244 and 3,780,128 describe batch oligomerization processes for .alpha.-olefins, such as 1-decene, using boron trifluoride in combination with a promoter such as an alcohol or water. Also, continuous oligomerization processes utilizing boron trifluoride catalysts are described in U.S. Pat. Nos. 4,045,508 and 4,239,930.
Oligomer mixtures typically comprised of dimer, trimer, tetramer, pentamer and minor amounts of higher oligomers are obtained from such processes. Whereas the distribution of oligomers can be varied to some extent, depending on the reaction conditions, it is not possible to totally exclude the formation of the dimeric product with boron trifluoride catalyzed reactions. The dimer, being too volatile (low flash point) for most lubricating applications, is therefore separated from the higher oligomeric products by distillation. In some instances, the higher oligomers are further fractionated to obtain products having different viscosity specifications. The higher oligomeric products (consisting predominantly of trimer and tetramer) are then hydrogenated to improve the oxidative and thermal stability prior to formulation of the finished lubricant product.
In view of the substantial quantity of dimer (2 centistoke) product which is presently being produced in commercial .alpha.-olefin oligomerization processes and the limited number of formulations for which 2 centistoke fluids are employed, the supply far exceeds demand. It would be highly advantageous if a process were available whereby dimer produced in boron trifluoride catalyzed oligomerization processes could be readily and economically converted to higher and more useful oligomers. The dimer fraction obtained from such boron trifluoride catalyzed oligomerizations consists predominantly of highly branched internal olefins (trialkyl- and tetraalkyl-substituted olefins) and, due to the fact that the unsaturation is located within the molecule and sterically hindered, is relatively unreactive and generally considered to be unacceptable for recycling in the process for further reaction and conversion to higher oligomers.
U.S. Pat. No. 4,172,855 to Shubkin et al. discloses a two-stage process whereby an .alpha.-olefin is first reacted to obtain a product consisting mainly of dimers of said .alpha.-olefins. Ziegler-type catalysts, preferably alkyl aluminum compounds, are utilized in the first stage of the process since catalysts of this type maximize dimer formation. In contradistinction to the highly branched internal olefins obtained from BF.sub.3 catalyzed oligomerizations, the dimers produced in the first stage of the Shubkin et al. process are primarily vinylidene-type ##STR1## products which are highly reactive as a result of the terminal methylene moiety. Thus, in the second stage of the process, the Shubkin et al. dimers can be further reacted with a second .alpha.-olefin to obtain the more desirable higher oligomers. A Friedel-Crafts catalyst, preferably a promoted boron trifluoride catalyst, is utilized to catalyze the second stage reaction of the dimer and .alpha.-olefin. Promoters disclosed by Shubkin et al. for use with the BF.sub.3 include water, alkanols, fatty acids, fatty acid esters, ethers and ketones.